RuntimeError: dictionary changed size during iteration (When using pyglet)

[CasperVM]

When using Pyglet instead of PyQt (which doesnt work and throws a segfault), The process finishes correctly and all blocks are exported. However, the GUI closes with the following traceback;

Traceback (most recent call last):
  File "/home/casper/envy/bin/blockcrafter-export", line 11, in <module>
    load_entry_point('blockcrafter', 'console_scripts', 'blockcrafter-export')()
  File "/home/casper/tmpedits/blockcrafter/blockcrafter/export.py", line 217, in main
    app.run()
  File "/home/casper/envy/lib/python3.4/site-packages/vispy/app/_default_app.py", line 62, in run
    return default_app.run()
  File "/home/casper/envy/lib/python3.4/site-packages/vispy/app/application.py", line 142, in run
    return self._backend._vispy_run()
  File "/home/casper/envy/lib/python3.4/site-packages/vispy/app/backends/_pyglet.py", line 159, in _vispy_run
    return pyglet.app.run()
  File "/home/casper/envy/lib/python3.4/site-packages/pyglet/app/__init__.py", line 138, in run
    event_loop.run()
  File "/home/casper/envy/lib/python3.4/site-packages/pyglet/app/base.py", line 142, in run
    self._run()
  File "/home/casper/envy/lib/python3.4/site-packages/pyglet/app/base.py", line 154, in _run
    timeout = self.idle()
  File "/home/casper/envy/lib/python3.4/site-packages/pyglet/app/base.py", line 278, in idle
    for window in app.windows:
  File "/home/casper/envy/lib/python3.4/site-packages/pyglet/app/__init__.py", line 100, in __iter__
    for key in self._dict.keys():
  File "/home/casper/envy/lib/python3.4/weakref.py", line 418, in keys
    for wr in self.data:
RuntimeError: dictionary changed size during iteration